| O'Brien (Barony) |
Murrough O'Brien - Wed 21st Oct 1654
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| of Offaly (Earldom) |
James Fitzgerald - Thu 19th Mar 1761
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- |
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| Ogle (Viscountcy) |
William Ogle - Sat 23rd Dec 1645
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| O'Grady (Barony) |
Standish O'Grady - Thu 6th Jan 1831
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| O'Neill (Barony) |
John O'Neill - Fri 25th Oct 1793
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| O'Neill (Viscountcy) |
John O'Neill - Sat 3rd Oct 1795
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| O'Neill (Earldom) |
Charles Henry O'Neill - Thu 28th Aug 1800
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| Ongley (Barony) |
Robert Henley Ongley - Tue 30th Jul 1776
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| Oranmore and Browne (Barony) |
Dominick Browne - Wed 4th May 1836
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| Oriel (Barony) |
Margaretta Amelia Foster - Sat 5th Jun 1790
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| of Ormond (Marquessate) |
Walter Butler - Fri 12th Jan 1816
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10 August 1820 |
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| of Ormond (Marquessate) |
James Wandesford Butler - Wed 5th Oct 1825
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25 October 1997 |
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| of Ormond (Dukedom) |
James Butler - Wed 30th Mar 1661
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| of Ormond (Marquessate) |
James Butler - Sat 30th Aug 1642
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| of Orrery (Earldom) |
Roger Boyle - Sun 5th Sep 1660
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| Orwell (Barony) |
Francis Vernon - Wed 7th Apr 1762
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| Orwell (Viscountcy) |
Francis Vernon - Sun 21st Jul 1776
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| Oxmantown (Barony) |
Richard Parsons - Wed 2nd Jul 1681
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| Oxmantown (Viscountcy) |
Laurence Harman Harman - Fri 2nd Oct 1795
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| Oxmantown (Barony) |
Laurence Harman Harman - Sat 22nd Sep 1792
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